path: root/_drafts/building-an-epaper-badge.md

---
layout: post
title: Building an ePaper badge
date: 2025-08-06 19:53 +0200
lang: en
categories: tech
---

## Foreword

Two weeks before [Awoostria]({% post_url 2025-07-30-awoostria-con-report %}):

> Hey, I should build something for my Tinkering Projects Show And Tell panel!

So it begins… The story how I built myself an ePaper badge.

Actually, the story begins way earlier, when I still had a physical Raspberry
Pi running stuff in my home network. I wanted to tinker around a bit and bought
myself a Waveshare ePaper. These are simple black-and-white displays which
maintain their content when the power goes off They are also inside eBook
readers.

Some years ago I wanted to build myself an electronic door sign for the EAST
convention with these, and I wanted to go "as minimal as possible". I wanted to
use one of the MSP430 controllers I had laying around, and I wanted to switch
motives via MiFare RFID transponders (using an MFRC5xx reader).  Work on that
development never really took off.

## Requirements

So this time, I simply said "fuck it", and threw an ESP32 on the problem.
Also, I decided to use [PlatformIO](https://platformio.org/), a
toolchain/SDK/library manager. I started with the Arduino framework, which is…
pretty wasteful in terms of resources (Flash/RAM/…), but speeds up development
significantly.

I had a simple ESP32 devboard, and one of the Waveshare modules, and started
coding.

…

But wait, what to I even want to achieve? Well, I wanted to "mood badge", i.e.
show my current mood with funny pictures. I couldn't get one on previous
conventions, so I was just gonna build one myself.

This involves several sub-problems:

* [Control the ePaper display](#control)
* [Get the pictures on the display](#pic)
* [Set what is displayed](#setdisp)
* [Power-saving](#powersave)
* [Attach the badge to myself](#attach)

<span id="control"></span>
## Control the display

Usually, you never talk to the displays themselves, but to a display
controller. Usually, you talk to these via a digital interface, e.g.  SPI.
There are different display controllers with different command sets.

But why bother with implementing this myself? There are ready-made libraries.
For myself, I decided to use [GxEPD2](https://github.com/ZinggJM/GxEPD2). They
support *some* Waveshare displays. The problem with Waveshare displays is, they
don't disclose which display controller they use. So it's kind of an
trial-or-error procedure. Or rather, you can look at their example code, figure
out which commands they are using, and compare what commands GxEPD2 uses.
That's a but cumbersome.

<span id="pic"></span>
## Get the picture on the display

You can't just simply throw a JPEG onto the display.  a) The display doesn't
understand that. It only understands pixel data.  (the library doesn't,
either).  b) The display can only draw black and white pixels.

So, you definitely can't throw a color picture on the display, not a monochrome
one. (There are displays which support a few gray-levels, though.)

So. What to? The solution is "dithering". I.e. you trick your eye into
perceiving grey by having clusters of black and white pixels. There is some
background to dithering, but I simply used either GIMP, the Floyd-something
algorithm, or one of the "ordered" modes of ImageMagick.

This, then, looks like this:

<img
  src="data:image/jpeg;base64,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"
  alt="a dithered image of my fursona"
/>

Now, about the image format… GxEPD2 supports "XBitmaps", or XBMs, which are
basically just a C array declaration, so you can GCC that file and throw in
error into the GxEPD2 function call. And voilá, it works.

<span id="attach"></span>
## How to attach the badge to myself

This would be cumbersome. I had a Waveshare module/PiHat (to heavy), and a
simple "ePaper sheet" - which would be too cumbersome to attach to the
devboard. So, at this point, I decided to switch from the prototyping platform
onto something better.

Fortunately, Elecrow provides a
[CrowPanel](https://www.elecrow.com/crowpanel-esp32-2-9-e-paper-hmi-display-with-128-296-resolution-black-white-color-driven-by-spi-interface.html),
which is exactly what I need. It has a display, a built-in ESP32, a housing,
and even some switches! As a huge plus, they even write which display
controller they use.  I had to try some of the GxEPD2 display classes, but
finally found one working. 

I decided to glue magnets onto the housing, and attach the display via magnets
on the inner side of my shirt - not ideal. I positioned the magnets in the
(vertical) middle of the housing, so it wobbles and is not readable. Also, I
accidentally washed the shirt after Awoostria with the magnets still sticking
inside - and now there's a hole in it. :( This problem is still unsolved.

<span id="setdisp"></span>
## Set what is displayed

In addition to these controls, which allow choosing the motive, I wanted
something "more direct", so I added the
[NimBLE-Arduino](https://github.com/h2zero/NimBLE-Arduino) library. With a bit
of coding, the available presets/motives could be read via BLE, and also one
could be selected via another characteristic. I started writing the
characteristic with nRF Connect For Mobile, but started an app later.

Actually, the switch-selection was a but troublesome.  Redrawing the whole
display takes around 2 seconds - but that is not acceptable when navigating the
presets one-by-one.  By looking at the API, I found out you can select a
"partial region".

What I didn't mention yet, the text that shows the mood is drawn using the API
- not integrated into the picture. So, I simply update a region in the
vertical-center-right of the display with the mood text.  The selection is
confirmed my pressing the rotary switch.

This is still not ideal. The first update must be a full-one, and I don't save
the selected preset in NVS - I don't want to destroy the flash by
hundreds/thousands of write cycles. I don't have a solution to this, yet. Maybe
I'm gonna integrate an microSD card (there's a slot for that!).  Gonna research
a good wear-levelling file system for that. Probably not FAT. That doesn't need
to be readable on the PC. (And even if, simple stuff can be written in FUSE).
Drawing the first motive would also be an option, but I don't like it that
much.

<span id="powersave"></span>
## Power saving

I first measured the current and was shocked. The whole thing draws around
80-100 mA. Not a big surprise, given that Arduino basically calls `loop()` over
and over again.

Using a bit of experimenting, and failing to cancel light sleep with an GPIO
interrupt, I  implemented power saving by using a timer-based light sleep (10
ms, gives good user response) and reducing the CPU frequency to 80 MHz.  And lo
and behold, my USB current measuring equipment (resolution 10mA) showed 0 mA.
Success.

Actually, reducing the CPU frequency was a requirement! If I didn't do that,
the CPU would constantly crash when entering or exiting light sleep. No idea
why!

Apropos of powering: The badge is normally unpowered and only powered if I
need to switch the motive. I don't want to have an USB cable hanging on me the
whole time.

<span id="theapp"></span>
## The app

Writing the BLE characteristics with nRF Connect is all and well, but not
really user-friendly. I didn't want to install the Android SDK, so I looked at
cloud based development for a start. I found [MIT
AppInventor](https://appinventor.mit.edu/).  First, I was disgusted, because
apparently they require Login with Google.  But I found [an alternative
way](https://code2.appinventor.mit.edu/) by which you simply get a
"Passphrase-like" codeword you use for login.

The graphical programming is unusual to be. I used Scratch in the past, so it
was not completely foreign. Actually, it was kinda fun coding this, in "event
style", once I figured out how to to like stripping, list filtering, etc.

… 

This was good enough for a while, but then I decided I wanted to actually
have the source code available. So I looked again at development options,
and settled for Flutter.

…

## Resources

- [surma.dev about dithering](https://surma.dev/things/ditherpunk/)
- [git repo with badge source code](https://git.uvok.de/espadge/)
- [git repo with app source code](https://git.uvok.de/espadge-flutter/)